Electronic power switching devices are widely used in many applications, such as, for example, motor controls, inverters, line switches, pulse circuits, and other power switching applications. A silicon controlled rectifier (SCR) or thyristor is a bistable semiconductor switching device formed from four layers of silicon. One type of power switching device, the MOS controlled thyristor (MCT) is especially suited for resonant (zero voltage or zero current switching) applications. The MCT has a forward voltage drop much like the SCR, and therefore enjoys greatly reduced conduction power loss. The MCT allows the control of high power circuits with very small amounts of input energy--a feature common to SCRs as well. In an MCT, turn-off is accomplished by turning on a highly interdigitated off-FET to short out one or both of the emitter-base junctions of a thyristor.
Another advantageous power switching device is the insulated gate bipolar transistor (IGBT) which is designed for high voltage, low on-dissipation applications, such as switching regulators and motor drivers. The IGBT can be operated from low power integrated circuits. The IGBT is also an insulated gate, field controlled switching device like the MCT. Available MCTs and IGBTs are useful at high switching frequency than is generally practice with power Darlington transistors, for example. In addition, both may be operated with junction temperatures of 150.degree. C. and above, and operate in switching circuits having 600 volts or higher switch ratings.
One approach to fabricating power switching devices involves direct semiconductor-semiconductor wafer bonding. The wafer bonding has been for the purpose of replacing a thick, e.g. 100 .mu.m epitaxial layer growth. For this bonding application, high temperature bonding anneals at temperatures of greater than about 1100.degree. C. are typically used to remove microvoids and bubbles. Both hydrophobic and hydrophilic bonding has been used.
Recently there has been increasing interest in the possibility of fabricating switching power devices with MOSFET current control devices on both the front side and back side of the power device to achieve faster turnoff of the device such as disclosed in U.S. Pat. No. 4,977,438 to Abbas. The conventional approach for fabricating double-sided MOSFET controlled power devices is to perform processing and photosteps on both sides of the wafer. This approach required critical control of thermal budgets, has approximately a factor of two increase in fabrication steps, and increases the possibility of yield loss due to scratches, etc.
U.S. Pat. No. 5,541,122 to Tu et al., for example, discloses a fabrication method for IGBT wherein two wafers are bonded together, and annealed at a temperature in a range of 800 to 1100.degree. C. An N-type wafer is doped N+ at a surface thereof and is bonded to a P+ wafer to define an N+ buffer region for the IGBT. Thereafter, a gate is formed on the upper surface and various diffusions are also made adjacent the gate to define an emitter/collector encircling the gate. An emitter contact is formed on the diffusions and a collector contact is deposited on the lower surface of the wafer using conventional techniques.
Unfortunately, the relatively high temperature annealing and subsequent device processing steps may adversely affect the doping profile of the buffer layer. Accordingly, the turnoff speed may be reduced. In addition, the double-sided processing after annealing requires a relatively large number of process steps, and the substrates are subject to mechanical damage which may reduce yields.